The invention relates to a hydrostatic variable displacement pump of swash plate construction which allows easier installation while using a compact housing.
Hydrostatic, closed-circuit variable displacement pumps of swash plate construction are provided with displacement pistons which are guided in cylinders and rotate about the shaft of the variable displacement pump. During the rotation, the displacement pistons are supported on the swash plate. With each 360xc2x0 rotation, each displacement piston executes a complete stroke.
The swash plate, which may be designed as an adjustable-angle plate or as a rocker device, forms a planar running surface for the displacement pistons. The swash plate is referred to as a rocker device if it is mounted in cylinder shells on rolling-contact elements and is pressed into the cylinder shells by means of suitable holding-down devices. The swash plate is referred to as an adjustable-angle plate if it can be pivoted about the bearing journals.
Machine elements mounted in rolling-contact bearings, e.g. shafts, are usually introduced axially, by way of their bearings, into the bearing seats in their respective housings. This presupposes that the largest diameter of the machine element which is to be mounted is smaller than the largest bearing seat in the housing, in order for it to be possible for the machine element to be installed axially by way of this bearing seat.
If the largest diameter, located between the bearings, of the machine element which is to be mounted is larger than the distance between the bearing seats in the housing, the machine element cannot be installed by way of the bearing seat. In transmission construction, the transmission housing is split in order for it to be possible to ensure appropriate installation. A split transmission housing, however, has a number of disadvantages, these residing, in particular, in a reduction in the structural rigidity and increased outlay in terms of sealing.
Swash plates of hydrostatic pumps often have a maximum dimension which is larger than the distance between the bearing seats. In such cases, it is necessary to provide in the pump housing an opening which is large enough for axial installation and in which, once the swash plate has been introduced into the housing, a type of housing cover is then fastened. The external diameter of the housing cover here is larger than the largest swash plate diameter located between is the bearings. This cover fits with its external diameter into the housing bore and then accommodates the one bearing of the adjustable-angle plate in its internal diameter. The housing of the variable displacement pump thus has to be of relatively large configuration or the rigidity of the housing is reduced by the large opening. In some existing variable displacement pumps, in order to install a swash plate around a shaft in the housing, it is necessary to have a large opening which can be closed off by means of a cover.
In yet another known hydrostatic variable displacement pump, the outer bearing races are inserted in bores of the adjustable-angle plate, the structural unit comprising adjustable-angle plate and outer bearing races being introduced into the pump housing through a sufficiently large opening at any desired location. Journals with the inner bearing constituent parts plugged thereon are then introduced laterally into the housing. These journals engage in the outer bearing races on the adjustable-angle plate. The journals are connected to the housing, with the result that the adjustable-angle plate is mounted in the pump housing such that it can be rotated about the journals. This requires a very high degree of accuracy in production. The cover, on which there is fitted a journal for accommodating a bearing, has to be fastened on the housing rather than being an integrated constituent part thereof.
If the largest diameter of the swash plate is larger than the largest bearing seat in the housing during installation there is increased outlay in terms of components in the form of housing covers or additional journal structures. They are more expensive to produce overall, which results in the accuracy having to meet more stringent requirements, and which have an adverse effect on the structural rigidity of the housing.
Also, when the adjustable-angle plate is installed axially, i.e. in the direction of the shaft of the variable displacement pump, the rolling-contact bearings are pushed into the bearing seats. The bearing seats correspond approximately to half-shells. These half-shells, in most cases, give a wrap angle around the outer bearing race of not more than 180xc2x0, because the outer bearing races can be positioned relatively easily in the bearing seat. If the bearings are pressed in radially by axial introduction into the bearing seat, it is also possible for the wrap angle to be just over 180xc2x0. A wrap angle of considerably less than 360xc2x0, however, is associated with the problem of it being possible for the bearing-supporting capacity to be reduced, and for the radial and axial fixing of the bearings often requiring additional design outlay. This is because, when the wrap angle in the bearing seat is smaller than 180xc2x0 or is only just over 180xc2x0, loading necessitates hold-down means in order to ensure reliable seating of the adjustable-angle plate in the bearing.
Therefore, a principal object of this invention is to provide a variable displacement pump by means of which an adjustable-angle plate of which the largest diameter or largest dimension is larger than the largest distance between its bearings can be installed in a compact non-split housing and can be mounted in commercially available 360xc2x0 bearings.
These and other objects will be apparent to those skilled in the art.
The invention provides a hydrostatic variable displacement pump of swash plate construction which has a cylinder block, arranged in a housing, with displacement pistons guided therein, a swash plate and at least a first bearing and a second bearing, which supports the swash plate in respect of bearing seats in the housing. The swash plate can have its angle position adjusted in relation to the movement direction of the displacement pistons by means of a servosystem. The largest dimension of the swash plate is larger than the distance between the bearing seats of the first bearing and of the second bearing. The housing has a first opening and a second opening, with the swash plate being introduced into the housing through the first opening. The second opening may be closed off by means of a cover, in which the servosystem for the angle adjustment of the swash plate is preferably integrated. The first bearing has a removable outer race which is designed such that, following installation of the swash plate in the bearing seat of the second bearing, it fixes the swash plate in the associated bearing seat of the first bearing.
This invention does away with the need for an additional cover, on which journals are fitted as a bearing seat, with the result that the two bearings for the swash plate are provided in the housing itself, which is designed as a non-split housing. It is thus possible for the production of the housing including the bearing seats to take place in one clamping setting, which is not only more straightforward in production terms, but considerably increases the production accuracy, in particular, of the two bearing seats in relation to one another. Since use can be made of a non-split housing, additional sealing problems with a cover which is to be provided in addition do not arise. Moreover, in terms of structural rigidity, a non-split housing has the advantage over a split housing with respect to the high hydraulic pressures occurring in the case of such variable displacement pumps. The possibility of using bearing seats with a wrap angle of considerably more than 180xc2x0 around the outer race of the bearing makes it possible to use adjustable-angle plates without hold-down devices. It is also possible for the hydrostatic variable displacement pump to be used with hold-down devices. By virtue of the bearing seats being fitted in a non-split housing, bearing failure and alignment errors between the bearing seats are thus reduced, if not avoided altogether. Overall, the variable displacement pump makes it possible to use a very compact housing and to reduce the number of necessary components.
According to one embodiment of the invention, the swash plate is designed as an adjustable-angle plate which has a servoarm which is preferably provided with an angled lever. This servoarm extends, beyond the external diameter of the bearing seat of the first bearing, into the servosystem, which closes off the opening in the housing, on which the first bearing is arranged. By virtue of the servoarm, which has the bearing projecting through it and is enclosed by the cover, the compactness of the variable displacement pump is further increased. On the other hand, the full structural rigidity of the subassembly is ensured.
The servoarm with its angled lever is preferably dimensioned and/or arranged such that the movement axis of the servopiston of the servosystem runs through the servoarm. This realizes relatively straightforward force transmission from the servopiston to the adjustable-angle plate without it being necessary to provide additional force-transmission devices.
The second bearing has a bearing seat which supports the second bearing over a circumferential region which is preferably considerably greater than 180xc2x0. As a result, during installation, the second bearing has to be introduced radially (transversely to the pump shaft) into the bearing seat. That is, installation of the bearing in the bearing seat in the axial direction (in the direction of the longitudinal axis of the pump shaft) is not possible. As a result, however, the supporting capacity of the bearing seat is considerably increased.
The swash plate is designed as a rocker device which is pushed into the bearings by means of a holding-down device. Using a rocker device as the swash plate exploits the advantages of a compact swash plate construction, consisting of a high level of possible bearing loading by the use of a bearing seat, located in an inner housing, of greater than 180xc2x0. Also use of commercially available rolling-contact bearings is possible. Such bearings are those which can absorb axial and radial forces, i.e. preferably tapered roller bearings.
The swash plate is formed in one piece. This reduces the number of components of the subassembly and also increases the structural strength of the swash plate, which is likewise subjected to high loading.